1. Field of the Invention
The present disclosure relates to temperature sensors, and more particularly to total air temperature sensors, such as used in aerospace applications.
2. Description of Related Art
Modern jet powered aircraft require very accurate measurement of outside air temperature for inputs to the air data computer, engine thrust management computer, and other airborne systems. For these aircraft types, their associated flight conditions, and the use of total air temperature probes in general, air temperature is better defined by the following four temperatures: (1) Static air temperature (SAT) or (TS), (2) total air temperature (TAT) or (Tt), (3) recovery temperature (Tr), and (4) measured temperature (Tm). Static air temperature (SAT) or (TS) is the temperature of the undisturbed air through which the aircraft is about to fly. Total air temperature (TAT) or (Tt) is the maximum air temperature that can be attained by 100% conversion of the kinetic energy of the flow. The measurement of TAT is derived from the recovery temperature (Tr), which is the adiabatic value of local air temperature on each portion of the aircraft surface due to incomplete recovery of the kinetic energy. Recovery temperature (Tr) is obtained from the measured temperature (Tm), which is the actual temperature as measured, and which can differ from recovery temperature because of heat transfer effects due to imposed environments.
Total air temperature sensors used at the inlets of gas turbine engines, for example, can use airfoil shaped members with slots positioned so the gas stream to be sensed passes through one of the slots, and the temperature sensor element is mounted in the slot. Examples of such systems are disclosed in U.S. Pat. No. 3,512,414 which is incorporated by reference herein in its entirety. Such sensor designs can mitigate the effects of high velocity foreign objects being ingested by the engine, and can include provisions for deicing.
One ongoing challenge for total air temperature measurements is associated with operation at higher Mach numbers. Compressibility effects occurring at higher Mach numbers can alter the desired flow pattern through traditional sensors, with potential reduction in response time, for example if there is reduced flow bathing the actual sensor element.
Another phenomenon which presents difficulties to some conventional TAT probe designs has to do with the problem of boundary layer separation, or “spillage,” at low mass flows. Flow separation creates two problems for the accurate measurement of TAT. The first has to do with turbulence and the creation of irrecoverable losses that reduce the measured value of TAT. The second is tied to the necessity of having to heat the probe in order to prevent ice formation during icing conditions. Anti-icing performance is facilitated by heater elements embedded in the housing walls. Unfortunately, external heating also heats the internal boundary layers of air which, if not properly controlled, provides an extraneous heat source in the measurement of TAT. This type of error, commonly referred to as deicing heater error (DHE), is difficult to correct for.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for systems and methods that allow for improved total air temperature sensor performance, including improved time response at elevated Mach numbers. There also remains a need in the art for such systems and methods that are easy to make and use. The present disclosure provides a solution for these problems.